A close associate of Donald Trump’s former national security adviser Michael Flynn arranged a covert investigation into Hillary Clinton’s use of a private email server when she was secretary of state, and through intermediaries turned to a person with knowledge of the “dark web” for help.

Flynn, a retired three-star general who led chants of “lock her up” at last year’s Republican national convention, is a central figure in the FBI’s investigation into whether the Kremlin worked with the Trump campaign to sway the US election.